Allow cli-eaa to ingest Jinja variable from environment variable

[bitonio]

feature originally requested by @alexkleinfig

Current status

Today cli-eaa supports passing variable (name, value) down to the application configuration template with Jinja2 syntax, using command line arguments like (more on the doc):

cat myapp.json.j2 | akamai eaa app create --var FOO bar --var --var QUX baz

It requires crafting some long command line.

Feature request

This feature request is to allow an equivalent with env vars such as:

export CLIEAA_VAR_FOO="bar"
export CLIEAA_VAR_QUX="baz"
cat myapp.json.j2 | akamai eaa app create